1. Technical Field of the Invention
The present invention relates to a method of short-term rejoining of a flight plan by radar guidance of an aircraft. It applies notably to the field of avionics, and more particularly to flight management devices usually designated according to the acronym FMS standing for the expression “Flight Management System”.
2. Discussion on the Background
Most current aircraft possess a flight management system, for example of the FMS type, according to the acronym of the term “Flight Management System”. These systems allow an aid to navigation, by displaying information useful to pilots, or else by communicating flight parameters to an automatic piloting system. Notably, a system of FMS type allows a pilot or some other qualified person, to enter, in pre-flight, a flight plan defined by a departure point, an arrival point, and a series of waypoints, usually designated by the abbreviation WPT. All these points can be chosen from among points predefined in a navigation database, and which correspond to airports, radionavigation beacons, etc. The points can also be defined by their geographical coordinates and their altitude. The entry of the waypoints can be done through a dedicated interface, for example a keyboard or a touchscreen, or else by transfer of data from an external device. A flight plan thus consists of a succession of segments, or “legs” according to the terminology usually employed in this technical field. Other data can be input into the flight management system, notably data relating to the aircraft's load plan and to the quantity of fuel on board. When the aircraft is in flight, the flight management system precisely evaluates the position of the aircraft and the uncertainty in this item of data, by centralizing the data originating from the various positioning devices, such as the satellite geo-positioning receiver, the radionavigation devices: for example DME, NDB and VOR, the inertial platform, etc. A screen allows the pilots to visualize the current position of the aircraft, as well as the track followed by it, and the closest waypoints, all on a map background making it possible to display other flight parameters and noteworthy points simultaneously. The information visualized allows notably the pilots to adjust flight parameters, such as heading, thrust, altitude, climb or descent rates, etc. or quite simply to monitor the proper progress of the flight if the aircraft is piloted automatically. The computer of the flight management system makes it possible to determine an optimal flight geometry, notably in the sense of a reduction in operating costs related to fuel consumption.
However, it frequently happens that an aircraft is constrained to leave the trajectory of a flight plan during the flight, for example following requests by the air traffic control bodies, either with the aim of circumventing an obstacle generated by unfavourable meteorological conditions, or simply with the objective of saving time or economizing on fuel consumption, etc. In such situations, it is necessary for the aircraft to rejoin the flight plan, from the instant at which the constraint no longer applies. Rejoining must take place in the most flexible possible manner, in terms of the aircraft's horizontal and vertical flight profiles, and speed profile; furthermore the choice of the waypoint to be rejoined must be relevant in this sense. It is furthermore desirable that the most realistic possible rejoining trajectory is determined, and taken into account by the FMS for the predictive calculations, notably of flight time and fuel consumption.
Moreover, as regards the guidance of the aircraft, two types of lateral navigation exist in current flight management systems. A first type of navigation is the controlled navigation mode or “NAV Mode” in which the flight of the aircraft is controlled by the FMS so as to follow the established lateral flight profile. A second type of lateral navigation is a mode of flight while holding a heading, or “HDG Mode” or else track mode or “TRK Mode”. For this second type of lateral navigation mode, that is to say for the HDG or TRK mode, when the aircraft is proceeding outside of its flight plan, the FMS is based on simplistic assumptions for rejoining the trajectory of the flight plan. For example, the rejoining can be determined as a great circle to the last waypoint of the flight plan which has not been reached, or by an immediate return to the “active leg” of the flight plan. In reality, the rejoining of such a point may be unrealistic, for example because it forces the aircraft to turn back, or else because it imposes trajectories on it which are unachievable in practice, having regard to the performance of the aircraft, the safety of the flight or else the comfort of its passengers. The predictions being made while taking such trajectories into account are then erroneous.
As regards vertical navigation, the vertical flight plan can be followed in flight according to the profile of the FMS, in a totally controlled mode, or else in a particular mode, for example holding a vertical speed or a speed at constant thrust. In a particular mode such as this, the guidance function of the FMS which formulates the target speeds is simplified and no longer adapts the speed of the aircraft as a function of the speed constraints which may be associated with the waypoints of the flight plan. This limitation induces late and fierce changes of speed of the aircraft, which most of the time are counterbalanced manually by the pilot, who must then take over the controls and exit the automatic piloting mode. Such manoeuvres are made to the detriment of flight safety and comfort, and may have unfortunate repercussions on fuel consumption. Furthermore, such manoeuvres monopolize the pilot's workload.
It should also be observed that a corollary of better controlled trajectories and speed profiles is less load not only for the flight staff, but also for the staff of the air traffic control bodies.